High speed low power magnetic devices based on current induced spin-momentum transfer

ABSTRACT

A high speed, low power method to control and switch the magnetization direction and/or helicity of a magnetic region in a magnetic device for memory cells using spin polarized electrical current. The magnetic device comprises a reference magnetic layer with a fixed magnetic helicity and/or magnetization direction and a free magnetic layer with a changeable magnetic helicity and/or magnetization direction. The fixed magnetic layer and the free magnetic layer are preferably separated by a non-magnetic layer. The fixed and free magnetic layers may have magnetization directions at a substantially non-zero angle relative to the layer normal. A current can be applied to the device to induce a torque that alters the magnetic state of the device so that it can act as a magnetic memory for writing information. The resistance, which depends on the magnetic state of the device, is measured to read out the information stored in the device.

This patent application is a continuation of U.S. patent applicationSer. No. 13/041,104, filed Mar. 4, 2011, which is a divisional of U.S.patent application Ser. No. 12/490,588, filed Jun. 24, 2009 (now U.S.Pat. No. 7,911,832), which is a continuation-in-part of U.S. patentapplication Ser. No. 11/932,745, filed Oct. 31, 2007 (now U.S. Pat. No.7,573,737), which is a continuation-in-part of U.S. patent applicationSer. No. 11/498,303, filed Aug. 1, 2006 (now U.S. Pat. No. 7,307,876),which is a continuation-in-part of U.S. patent application Ser. No.11/250,791 (now U.S. Pat. No. 7,170,778), filed Oct. 13, 2005, which isa continuation of U.S. patent application Ser. No. 10/643,762 filed Aug.19, 2003 (now U.S. Pat. No. 6,980,469) each of which is incorporatedherein by reference.

GOVERNMENT RIGHTS

This invention was made with government support under Contract NumberNSF-DMR-0706322 entitled “Spin Transfer in Magnetic Nanostructures” andNSF-PHY-0601179 entitled “Noise-Induced Escape in Multistable Systems”awarded by the National Science Foundation, and Contract NumberARO-W911NF-07-1-0643 entitled “Electronics: Ultra-Fast MagnetoelectronicDevices” awarded by the Army Research Office. The government has certainrights in the invention.

FIELD OF THE INVENTION

The present invention generally relates to magnetic devices used inmemory and information processing applications, such as giantmagnetoresistance (GMR) devices. More specifically, the presentinvention describes a high speed and low power method by which a spinpolarized electrical current can be used to control and switch thedirection of magnetization and/or helicity of a magnetic region in sucha device.

BACKGROUND OF THE INVENTION

Magnetic devices that use a flow of spin-polarized electrons are ofinterest for magnetic memory and information processing applications.Such a device generally includes at least two ferromagnetic electrodesthat are separated by a non-magnetic material, such as a metal orinsulator. The thicknesses of the electrodes are typically in the rangeof 1 nm to 50 nm. If the non-magnetic material is a metal, then thistype of device is known as a giant magnetoresistance or spin-valvedevice. The resistance of the device depends on the relativemagnetization orientation of the magnetic electrodes, such as whetherthey are oriented parallel or anti-parallel (i.e., the magnetizationslie on parallel lines but point in opposite directions). One electrodetypically has its magnetization pinned, i.e., it has a higher coercivitythan the other electrode and requires larger magnetic fields orspin-polarized currents to change the orientation of its magnetization.The second layer is known as the free electrode and its magnetizationdirection can be changed relative to the former. Information can bestored in the orientation of this second layer. For example, “1” or “0”can be represented by anti-parallel alignment of the layers and “0” or“1” by parallel alignment. The device resistance will be different forthese two states and thus the device resistance can be used todistinguish “1” from “0.” An important feature of such a device is thatit is a non-volatile memory, since the device maintains the informationeven when the power is off, like a magnetic hard drive. The magnetelectrodes can be sub-micron in lateral size and the magnetizationdirection can still be stable with respect to thermal fluctuations.

In conventional magnetic random access memory (MRAM) designs, magneticfields are used to switch the magnetization direction of the freeelectrode. These magnetic fields are produced using current carryingwires near the magnetic electrodes. The wires must be small incross-section because memory devices consist of dense arrays of MRAMcells. As the magnetic fields from the wires generate long-rangemagnetic fields (magnetic fields decay only as the inverse of thedistance from the center of the wire) there will be cross-talk betweenelements of the arrays, and one device will experience the magneticfields from the other devices. This cross-talk will limit the density ofthe memory and/or cause errors in memory operations. Further, themagnetic fields generated by such wires are limited to about 0.1 Teslaat the position of the electrodes, which leads to slow device operation.Importantly, conventional memory designs also use stochastic (random)processes or fluctuating fields to initiate the switching events, whichis inherently slow and unreliable (see, for example, R. H. Koch et al.,Phys. Rev. Lett. 84, 5419 (2000)).

In U.S. Pat. No. 5,695,864 and several other publications (e.g., J.Slonckewski, Journal of Magnetism and Magnetic Materials 159, L1(1996)), John Slonckewski described a mechanism by which aspin-polarized current can be used to directly change the magneticorientation of a magnetic electrode. In the proposed mechanism, the spinangular momentum of the flowing electrons interacts directly with thebackground magnetization of a magnetic region. The moving electronstransfer a portion of their spin-angular momentum to the backgroundmagnetization and produce a torque on the magnetization in this region.This torque can alter the direction of magnetization of this region andswitch its magnetization direction. Further, this interaction is local,since it only acts on regions through which the current flows. However,the proposed mechanism was purely theoretical.

Slonckewski's patent describes MRAM devices that use spin-momentumtransfer for magnetic switching. However, the proposed devices are slowand rely on fluctuating magnetic fields and stochastic processes toinitiate magnetization switching. Further, large current densities areneeded to switch the devices. In describing the preferred embodiment ofhis “latch or logic gate,” Slonckewski states “ . . . the preferred axesof the 3 magnets F1, F2, and F3 are all “vertical” (i.e., in the samedirection or orientation) as discussed above. Other orientations canserve as long as they are parallel to the same axis.” As we describebelow, our device makes use of layer magnetizations that are notparallel to the same axis, to great advantage in speed, reliability, andpower consumption.

U.S. Pat. No. 6,256,223 to Jonathan Sun also describes devices that usecurrent-induced magnetic switching and demonstrates in experiment theoperation of such devices. However, the devices proposed wereunreliable, as there was little consistency with regard to devicecharacteristics. Further, the estimated time scale for magneticswitching was 50 nsec for operation at large current densities.

Devices are needed that exhibit high speed and reliable operation underthe action of a spin-polarized current. This includes devices thatoperate with lower power and have lower threshold currents for switchingthe magnetization orientation.

SUMMARY OF THE INVENTION

In view of the limitations associated with conventional designs ofdevices that use spin-momentum transfer, an object of the presentinvention is to provide a structure that is optimal for a magneticmemory or magnetic information processing device.

It is another object of the present invention to produce a magneticdevice that has advantages in terms of speed of operation.

It is a further object of the present invention to produce a magneticdevice that has advantages in terms of reliability.

It is a further object of the present invention to produce a magneticdevice that requires lower power to operate.

It is a further object of the present invention to produce a magneticdevice that has advantages in terms of the stability of the storedinformation.

It is a further object of the present invention to produce a magneticdevice that has a large read-out signal.

These and additional objects of the invention are accomplished by adevice that employs magnetic layers in which the layer magnetizationdirections do not lie along the same axis. For instance in oneembodiment, two magnetic regions have magnetizations that areorthogonal.

The invention is a magnetic device comprised of ferromagnetic andnon-magnetic layers through which current can flow. The magnetic deviceis comprised of a ferromagnetic layer with a fixed magnetizationdirection and another ferromagnetic layer separated from the first by anon-magnetic region that has a magnetization that is free to rotate inresponse to applied currents. A third ferromagnetic layer, again,separated from the others by a non-magnetic layer, has a fixedmagnetization direction and can be employed to readout the magnetizationdirection of the free ferromagnetic layer. The magnetization directionsof the ferromagnetic layers are not all along the same axis. In one ofthe preferred embodiments, the first fixed ferromagnetic layer'smagnetization direction is perpendicular to the plane of the layer,while the free ferromagnetic layer's magnetization is in the plane ofthe layer. As described above, a current flow between the layerstransfers spin-angular momentum from the fixed magnetization layer tothe free magnetization layer and produces a torque on the magnetizationof the free layer. The torque is proportional to the vector tripleproduct of the magnetization direction of the fixed and free layer, witha factor of proportionality that depends on the current and the spinpolarization of the current. A large torque is produced when themagnetization directions of the fixed and free layers are orthogonal.

This large torque acting on the magnetization direction of the freemagnetic layer causes the magnetization of the free magnetic layer torotate out of the plane of the layer. Since the thickness of the freemagnetic layer is less than the width and length dimensions, therotation of the magnetization of the free magnetic layer out of theplane of the layer generates a large magnetic field, a ‘demagnetizing’field, which is perpendicular to the plane of the layer.

This demagnetizing field forces the magnetization vector of the freemagnetic layer to precess, i.e., for the magnetization direction torotate around the direction of the demagnetization magnetic field. Thedemagnetizing field also determines the rate of precession. A largedemagnetizing field results in a high precession rate, which is anoptimal condition for fast magnetic switching. An advantage of thismagnetic device is that random fluctuating forces or fields are notnecessary to initiate or control the magnetic response of the layers.

A further aspect of the invention provides a magnetic device including areference magnetic layer having a fixed magnetic helicity and/or a fixedmagnetization direction, a free magnetic layer with at least onemagnetization vector having a changeable magnetization helicity, andnon-magnetic layer spatially separating said free magnetic layer andsaid reference magnetic layer. The magnetization helicity of the freemagnetic layer can be changed using current induced spin-momentumtransfer. In one preferred embodiment, the device has a substantiallyring shaped structure, and the reference magnetic layer includes an easyaxis substantially perpendicular to the reference layer and a fixedmagnetization perpendicular to the plane of the reference layer.Alternatively, the reference layer includes an easy axis substantiallyperpendicular to the reference layer and a magnetic helicitysubstantially clockwise or counter-clockwise about the ring-shapedstructure.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description and drawings ofthe illustrative embodiments of the invention wherein like referencenumbers refer to similar elements throughout the views and in which:

FIG. 1 is an illustration of a magnetic device according to the presentinvention;

FIGS. 2A-2E are illustrations of the free magnetic layer showing themagnetization vector and the demagnetizing field of the electronicdevice of FIG. 1 during the application of pulses of current asillustrated in FIG. 3A;

FIG. 3A is an illustration of a current waveform that may be applied tothe magnetic device;

FIG. 3B is an illustration of an alternate current waveform that may beapplied to the magnetic device;

FIG. 4 is an illustration of a memory cell according to one embodimentof the present invention;

FIGS. 5A-5E are illustrations of the free magnetic layer showing themagnetization vector and the demagnetizing field of the memory cell ofFIG. 4;

FIG. 6A is an illustration of a current waveform that may be applied tothe memory cell of FIG. 4 during a write operation;

FIG. 6B is an illustration of a resistance measured from the memory cellduring a read-out operation before and after the current pulse shown inFIG. 6A is applied;

FIG. 7 is an illustration of the free magnetic layer of a 4-state memorycell;

FIG. 8 is an illustration of an example of a current waveform applied tothe magnetic device;

FIG. 9 is an illustration of the magnetization components of the freemagnetic layer during and after the application of the current pulseshown in FIG. 8;

FIG. 10 is an illustration of a memory cell according to one embodimentof the present invention in which during writing operations no netcurrent passes through the free magnetic layer;

FIG. 11 is an illustration of an annular magnetic device according tothe present invention;

FIG. 12 is an illustration of an annular memory cell according to oneembodiment of the present invention;

FIG. 13 is an illustration of an annular memory cell according to afurther embodiment of the present invention in which separate read andwrite contacts are provided;

FIGS. 14A-D are illustrations of the response of the resistance of amagnetic device to current pulses of variable length, according toembodiments of the present invention;

FIGS. 15A-D are illustrations of magnetic devices having either pinnedor free layer magnetizations oriented at substantially non-zero anglesrelative to the layer normal;

FIG. 16 is an illustration of a method of making a magnetic layer havinga magnetization oriented at a substantially non-zero angle relative tothe layer normal;

FIG. 17A is an illustration of the magnetization direction of an annularmagnetic device having a constant saddle configuration;

FIGS. 17B-C are illustrations of the magnetization directions of annularmagnetic devices having instanton saddle configurations;

FIG. 18 is an illustration of the energy barrier to magnetizationreversal of an annular magnetic device as a function of ring radius forvarious ring thicknesses;

FIG. 19 is an illustration of the figure of merit of an annular magneticdevice as a function of ring size in the constant and instanton saddleregimes; and

FIG. 20 is an illustration of a memory architecture constructed fromannular magnetic devices according to embodiments of the presentinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Structure of a Basic Magnetic Device

To illustrate the basic concept, FIG. 1 shows a multilayered,pillar-shaped magnetic device comprising a pinned magnetic layer FM1with a fixed magnetization direction and a free magnetic layer FM2 witha free magnetization direction. {right arrow over (m)}₁ Is themagnetization vector of the pinned magnetic layer FM1, and {right arrowover (m)}₂ is the magnetization vector of the free magnetic layer FM2.The pinned magnetic layer FM1 acts as a source of spin angular momentum.

The pinned magnetic layer FM1 and the free magnetic layer FM2 areseparated by a first non-magnetic layer N1 that spatially separates thetwo layers FM1 and FM2 such that their mutual magnetic interaction isminimized. The pillar-shaped magnetic device is typically sized innanometers, e.g., it may be less than approximately 200 nm laterally.

The free magnetic layer FM2 is essentially a magnetic thin film elementimbedded in a pillar-shaped magnetic device with two additionallayers—the pinned magnetic layer FM1 and the non-magnetic layer N1. Thelayer thicknesses are typically approximately 1 nm to 50 nm.

These pillar-shaped magnetic devices can be fabricated in a stackedsequence of layers by many different means, including sputtering,thermal and electron-beam evaporation through a sub-micron stencil mask.These magnetic devices can also be fabricated in a stack sequence usingsputtering, thermal and electron-beam evaporation to form a multilayeredfilm followed by a subtractive nanofabrication process that removesmaterials to leave the pillar-shaped magnetic device on a substratesurface, such as that of a silicon of other semiconducting or insulatingwafer.

Materials for the ferromagnetic layers include (but are not limited to)Fe, Co, Ni, and alloys of these elements, such as Ni_(1-x)Fe_(x); alloysof these ferromagnetic metals with non-magnetic metals, such as Cu, Pd,Pt, NiMnSb, at compositions in which the materials are ferromagneticallyordered at room temperature; conducting materials; and conductingmagnetic oxides such as CrO₂ and Fe₃O₄. For the nonmagnetic layers,materials include (but are not limited to) Cu, Cr, Au, Ag, and Al. Themain requirement for the non-magnetic layer is the absence of scatteringof the electron spin-direction on a short length scale, which is lessthan about the layer thickness.

An electric current source is connected to the pinned magnetic layer FM1and the free magnetic layer FM2 so that an electric current I cantraverse the pillar device.

Method of Magnetic Switching

An electric current I is applied to the pillar-shaped magnetic device sothat the current I flows through the various layers of the device, fromthe pinned magnetic layer FM1 to the first non-magnetic layer N1 to thefree magnetic layer FM2. The applied current I results in a transfer ofangular momentum from the pinned magnetic layer FM1 to the free magneticlayer FM2. As stated above, a transfer of angular momentum from onemagnetic region to another can produce a torque.

FIGS. 2A-2E show steps in the method of magnetic switching using themagnetic device shown in FIG. 1 and for convenience, FIGS. 2A-2E onlyshow the free magnetic layer FM2 and the magnetization vector {rightarrow over (m)}₂ of the free magnetic layer FM2. FIG. 2A shows theinitial state of the free magnetic layer FM2 before the current I isapplied.

As shown in FIGS. 2B-2D, applying a current I, which can be of a form asshown in FIGS. 3A and 3B, results in the transfer of angular momentumfrom the pinned magnetic layer FM1 to the free magnetic layer FM2. Thistransfer of angular momentum from the pinned magnetic layer FM1 to thefree magnetic layer FM2 produces a torque {right arrow over (τ)}_(S) onthe magnetic moment of the free magnetic layer FM2.

The torque {right arrow over (τ)}_(S) per unit magnetization of the freelayer is proportional to the vector triple product a_(I){circumflex over(m)}₂×({circumflex over (m)}₂×{circumflex over (m)}₁), where {circumflexover (m)}₂ is a unit vector in the direction of the magnetic moment ofthe free magnetic layer FM2 and {circumflex over (m)}₁ is a unit vectorin the direction of the magnetic moment of the pinned magnetic layerFM1. The prefactor, a_(I), depends on the current I, thespin-polarization P of the current I, and the cosine of the anglebetween the free and pinned magnetic layers, cos(θ), such that a_(I)=hIg(P,cos(θ))/(eMV). h is the reduced Planck's constant, g is a functionof the spin-polarization P and cos(θ), M is the magnetization density ofthe free layer, e is the charge of the electron, and V is the volume ofthe free layer (see, J. Slonczewski, Journal of Magnetism and MagneticMaterials 159, L1 (1996)). Thus, a large torque {right arrow over(τ)}_(S) is produced when the magnetic moments of the pinned magneticlayer FM1 and the free magnetic layer FM2 are perpendicular.

This torque {right arrow over (τ)}_(S), which acts on the magneticmoment of the free magnetic layer FM2, causes the magnetization of thefree magnetic layer FM2 to rotate out of the plane of the layer. Sincethe thickness of the free magnetic layer FM2 is less than the width andlength dimensions of the free magnetic layer FM2, the rotation of themagnetization vector {right arrow over (m)}₂ of the free magnetic layerFM2 out of the plane of the layer generates a large magnetic field, a‘demagnetizing’ field, which is perpendicular to the plane of the layer.

This demagnetizing field forces the magnetization vector {right arrowover (m)}hd 2 of the free magnetic layer FM2 to precess, i.e., to movesuch that the magnetization direction rotates about the magnetic fieldaxis. The demagnetizing field also determines the rate of precession. Alarge demagnetizing field results in an extremely high precession rate,which is an optimal condition for fast magnetic switching.

Thus, in an optimal configuration of the magnetic memory device for fastmagnetic switching, the magnetic moment of the pinned magnetic layer FM1is perpendicular to the plane of the free magnetic layer FM2, and themagnetic moment of the free magnetic layer FM2 is perpendicular to theaxis of the pillar of thin layers and lies in the plane of the freemagnetic layer FM2.

FIG. 2E shows the free magnetic layer FM2 after the magnetic switchingprocess is completed. As shown in FIGS. 2A and 2E, the magneticswitching process causes the magnetization vector {right arrow over(m)}hd 2 of the free magnetic layer FM2 to switch by reversing directionby rotating 180°.

FIGS. 3A and 3B show two different forms of current input that may beapplied to the magnetic device. The current input shown in FIG. 3A iscomprised of two current pulses of short duration, a first positivecurrent pulse followed by a second negative current pulse. This form ofcurrent input results in writing a ‘1’ or a ‘0’. Alternatively, thefirst current pulse can be negative and the second current pulse can bepositive, as long as the two current pulses are of opposite polarity. Inboth cases, the state of the magnetic bit will be changed from ‘1’ to‘0’ or ‘0’ to ‘1’ (i.e., the final state will be the complement of theinitial state of the bit). The current input shown in FIG. 3A is used inthe method of magnetic switching described above and shown in FIGS.2A-2E. Using a current input formed of two current pulses results in afaster magnetic switching process.

The first current pulse starts the precession of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2. After thecompletion of the first current pulse, the second current pulse isapplied to stop the precession at a desired state.

The second current pulse is not essential to the operation of thedevice, but it enables higher speed switching. For example, the currentinput shown in FIG. 3B is comprised of a single positive current pulse.Alternatively, a single negative current pulse may also be applied tothe magnetic device. Simulations show that many different types ofcurrent pulses switch FM2. Therefore device operation is certainly notlimited to the current pulses shown in FIG. 3.

Structure of a Memory Cell

The magnetic device described above can be incorporated into a memorycell for inclusion into arrays of memory cells to make up a magneticmemory. According to one embodiment as shown in FIG. 4, the magneticdevice of the present invention, when implemented as a memory cell, is amultilayered, pillar-shaped device having a pinned magnetic layer FM1with a fixed magnetization direction, a free magnetic layer FM2 with afree magnetization direction, and a read-out magnetic layer FM3 with afixed magnetization direction. {right arrow over (m)}₁ is themagnetization vector of the pinned magnetic layer FM1, {right arrow over(m)}₂ is the magnetization vector of the free magnetic layer FM2, and{right arrow over (m)}₃ is the magnetization vector of the read-outmagnetic layer FM3.

The pinned magnetic layer FM1 and the free magnetic layer FM2 areseparated by a first non-magnetic layer N1 that spatially separates thetwo layers FM1 and FM2 such that their mutual magnetic interaction isminimized. The free magnetic layer FM2 and the read-out magnetic layerFM3 are separated by a second non-magnetic layer N2 that spatiallyseparates the two layers FM2 and FM3 such that their mutual magneticinteraction is minimized. The pillar-shaped magnetic device is typicallysized in nanometers, e.g., it may be less than approximately 200 nm.

An electric current source is connected to the pinned magnetic layer FM1and the read-out magnetic layer FM3 so that an electric current I cantraverse the pillar device. A voltmeter is connected to the pinnedmagnetic layer FM1 and the read-out magnetic layer FM3 so that theresistance of the magnetic device can be measured to thereby read thelogical contents of the memory cell.

Method for Writing Information

The magnetic switching process is used when information is written intoa memory cell. To store a logical bit of information in a memory cell,the magnetization direction of the magnetization vector inside thememory cell is set in one of two possible orientations to code thelogical values of ‘0’ and ‘1’. This magnetic device, when implemented asa memory cell, uses the method of magnetic switching describedpreviously in order to store bits of information. Current pulses areapplied to change the logical value in the magnetic device. The magneticmemory device described above and shown in FIG. 4 stores one bit ofinformation since the free magnetic layer FM2 has a single magnetizationvector {right arrow over (m)}₂ with two stable magnetic states.

An electric current I is applied to the pillar-shaped magnetic memorydevice so that the current I flows through the various layers of themagnetic memory device, from the pinned magnetic layer FM1 to theread-out magnetic layer FM3. The applied current I results in a transferof angular momentum from the pinned magnetic layer FM1 to the freemagnetic layer FM2.

FIGS. 5A-5E show steps in the method of writing information using themagnetic memory device shown in FIG. 4 and for convenience, FIGS. 5A-5Eonly show the free magnetic layer FM2 and the magnetization vector{right arrow over (m)}₂ of the free magnetic layer FM2. FIG. 5A showsthe initial state of the free magnetic layer FM2 before the current I isapplied.

As shown in FIGS. 5B-5D, applying a current I, which can be of a form asshown in FIGS. 3A and 3B, results in the transfer of angular momentumfrom the pinned magnetic layer FM1 to the free magnetic layer FM2. FIGS.2A-2E and 5A-5E show the change in the orientation of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2 as aresult of applying the current to the magnetic device.

FIG. 6A shows a form of the current input that is applied to themagnetic memory device shown in FIG. 4. The current input of FIG. 6Aincludes two current pulses of short duration, a first positive currentpulse followed by a second negative current pulse, which results inwriting a ‘1’ or a ‘0’. Alternatively, the first current pulse can benegative and the second current pulse can be positive, as long as thetwo current pulses are of opposite polarity. In both cases, the state ofthe magnetic bit will be changed from ‘1’ to ‘0’ or ‘0’ to ‘1’ (i.e.,the final state will be the complement of the initial state of the bit).

The first current pulse starts the precession of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2. After thecompletion of the first current pulse, the second current pulse isapplied to stop the precession at a desired state. For this embodimentof the magnetic memory device of the present invention, the precessionis stopped when 180° rotation of the magnetization vector {right arrowover (m)}₂ of the free magnetic layer FM2 is achieved.

FIG. 6B shows an example of the corresponding resistance of the deviceas measured by the voltmeter connected to the magnetic memory deviceshown in FIG. 4 with a small current applied, i.e., a current intensitymuch less than that used in the current pulses. The resistance increasesafter the current pulses of FIG. 6A are applied to the device. At theinitial state shown in FIG. 5A (before the first positive currentpulse), the resistance is at a constant low value. At the final stateshown in FIG. 5E, the resistance is at a constant high value.

Thus, the states shown in FIGS. 5A and 5E correspond to a logical valueof “0” in the initial state and a logical value of “1” in the finalstate, respectively. The magnetization vector {right arrow over (m)}₂ ofthe free magnetic layer FM2 in the final state shown in FIG. 5E is inthe opposite direction than the magnetization vector {right arrow over(m)}₂ of the free magnetic layer FM2 in the initial state shown in FIG.5A.

The current pulse that is used to switch the magnetization vector {rightarrow over (m)}₂ of the free magnetic layer FM2 may have a minimum pulseduration that is required in order to switch the magnetization vector{right arrow over (m)}₂ between stable states. However, there isgenerally no maximum pulse duration; i.e., a current pulse will switchthe state of the magnetization vector {right arrow over (m)}₂ betweenstable states so long as it is applied for the minimum pulse duration,regardless of the extent to which the applied pulse duration exceeds theminimum. FIGS. 14A and 14B demonstrate this phenomenon for a pulse ofpositive polarity. FIG. 14A illustrates a current pulse that is appliedfor at least as long as the minimum pulse duration for switching,Δt_(min), and is then applied for a variable additional length of timerepresented by the dashed line. FIG. 14B plots the resistance of thedevice over time in response to the pulse of FIG. 14A. These figuresdemonstrate that, so long as the pulse is applied for at least Δt_(min),the device will switch from its initial, low-resistance stable state toits final, high-resistance stable state, regardless of the length of theadditional, variable-length pulse period. FIGS. 14C and 14D demonstratethis phenomenon for a pulse of negative polarity, which switches thedevice resistance from a high-resistance state to a low-resistancestate. A person of ordinary skill in the art would recognize that theabsolute polarity of the pulse used to switch the device resistance fromhigh to low or low to high is not important, so long as the pulses usedto change the device resistance from high to low and low to high are ofopposite polarity. Therefore, a pulse of “positive” absolute polaritycould be used to switch the resistance from high to low, while a pulseof “negative” absolute polarity could be used to switch the resistancefrom low to high.

The necessary amplitude of the current pulses can be estimated bynumerical modeling using the equations of micromagnetics, theLandau-Lifzshitz Gilbert equations including the spin-transfer torquediscussed earlier (see, for example, B. Oezyilmaz et al., Phys. Rev.Lett. 91, 067203 (2003)). For a free layer comprised of Co with amagnetization density of M=1400 emu/cm³, a Gilbert damping parameter αof 0.01, a spin-polarization of the current P of 0.4, and an in-planeuniaxial anisotropy field of 1000 kOe. (In this case, the in-planeuniaxial anisotropy constant K is K=7×10⁵ erg/cm³.) For the purposes ofthis estimation, the Co free layer is 3 nm thick and has lateraldimensions of 60 nm by 60 nm. We find that a current pulse of amplitudeof 5 mA is more than sufficient to switch the layer. The currentnecessary to switch the device is reduced by decreasing the size of theCo free layer; increasing the spin-polarization of the current, forexample, by using a pinned layer with a higher degree ofspin-polarization; and decreasing the in-plane anisotropy or decreasingthe Gilbert damping. For this current amplitude, a 35 psec pulse issufficient to switch the device.

With a device resistance of 5 Ohms, the energy dissipation is 5×10⁻¹⁵ J.This energy dissipation value can be compared to the energy needed toswitch a magnetic device with a spin-polarized current when the pinnedlayer and the free layer magnetizations are initially aligned along thesame axis. Recent experiments show that this requires a current ofapproximately 10 mA applied for approximately 10 ns in a device with aresistance of 5 Ohms (see, R. H. Koch et al. Phys. Rev. Lett. 92, 088302(2004)). The energy dissipated is thus 5×10⁻¹² J. Thus, in comparison,the power requirement for our device is quite small. Further, becausethe pulse is on only very briefly, in spite of the large currentdensities, 1 A/μm², no electromigration is expected. Further, we haveoperated such devices at current densities 5 times greater than thisvalue for extended periods (approximately 1 minute) with no devicedamage (see, B. Oezyilmaz et al., Phys. Rev. Lett. 91, 067203 (2003)).

Method for Reading Information

The read-out magnetic layer FM3 is required in the simplestimplementation of the magnetic memory device. The read-out magneticlayer FM3 has a magnetization vector {right arrow over (m)}₃ with afixed magnetization direction. The magnetization vector {right arrowover (m)}₃ of the read-out magnetic layer FM3 can be fixed in a numberof ways. For example, the read-out magnetic layer FM3 can be formedthicker or of a higher anisotropic magnetic material or can be placedadjacent to an antiferromagnetic layer to use the phenomena of exchangebiasing. In the phenomena of exchange biasing, the coupling between theantiferromagnetic layer and the ferromagnetic layer and the largemagnetic anisotropy of the antiferromagnetic layer results in ahardening of the ferromagnetic layer so that larger magnetic fields andcurrents are required to change its magnetization direction.

The resistance of the magnetic memory device is very sensitive to therelative orientation of the magnetization vector {right arrow over (m)}₂of the free magnetic layer FM2 and the magnetization vector {right arrowover (m)}₃ of read-out magnetic layer FM3. The resistance of themagnetic memory device is highest when the magnetization vectors {rightarrow over (m)}₂ and {right arrow over (m)}₃ of the free magnetic layerFM2 and the read-out layer FM3, respectively, are in anti-parallelalignment. The resistance of the magnetic device is lowest when themagnetization vectors {right arrow over (m)}₂ and {right arrow over(m)}₃ of the layers FM2 and FM3, respectively, are in parallelalignment. Thus, a simple resistance measurement can determine theorientation of the magnetization vector {right arrow over (m)}₂ of thefree magnetic layer FM2.

The fixed orientation of the magnetization vector {right arrow over(m)}₃ of the read-out magnetic layer FM3 is set so that it is either inparallel or anti-parallel alignment with the magnetization vector {rightarrow over (m)}₂ of the free magnetic layer FM2, depending on theorientation of the magnetization vector {right arrow over (m)}₂ of thefree magnetic layer FM2. Since the orientation of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2 switchesso that it can be rotated 180°, the magnetization vectors {right arrowover (m)}₂ and {right arrow over (m)}₃ of the free magnetic layer FM2and the read-out layer FM3, respectively, must be in eitheranti-parallel or parallel alignment.

Storage of Multiple Bits of Information

The magnetic memory device described above and shown in FIG. 4 has twostable magnetic states and is able to store one bit of information.According to another embodiment of the present invention, a magneticmemory device can be constructed to store multiple bits of information.FIG. 7 shows an example of a free magnetic layer FM2 with four stablemagnetic states. A magnetic memory device comprising a free magneticlayer FM2 with four stable magnetic states is able to store two bits ofinformation. In this embodiment, current pulses are applied to switchthe magnetization between directions that differ by 90° instead of 180°.This can be accomplished by current pulses of a different form. Forexample, the current pulses can be smaller in amplitude and/or shorterin duration. The readout layer (FM3) is then aligned such that each ofthe four magnetization states has a different resistance. This requiresthat the readout layer magnetization not have an in-plane component thatpoints parallel to any of the four states nor at 45° to these states.

EXAMPLE

The operation of the magnetic device was simulated usingLandau-Lifzshitz Gilbert equations including a spin-transfer torque.

FIG. 8 shows the amplitude of the current input applied to the magneticmemory device starting at an initial time t=0 and ending at t=30picoseconds. This current input comprises two current pulses similar tothe current input shown in FIGS. 3A and 6A.

A 16-picosecond positive current pulse is applied to the magnetic memorydevice to start the precession of the magnetization vector {right arrowover (m)}₂ of the free magnetic layer FM2. After this 16-picosecondcurrent pulse, a 14-picosecond negative current pulse is applied to themagnetic memory device to stop the precession of the magnetizationvector {right arrow over (m)}₂ of the free magnetic layer FM2 to achievea desired state of the magnetization vector {right arrow over (m)}₂. Formagnetic memory devices, the precession is stopped after achieving a180° rotation of the magnetization vector {right arrow over (m)}₂ of thefree magnetic layer FM2.

FIG. 9 shows the magnetization components m_(X) and m_(Y) of themagnetization vector {right arrow over (m)}₂ of the free magnetic layerFM2 in the x- and y-directions shown in FIGS. 2B and 5B. Themagnetization components m_(X) and m_(Y) are measured during and afterthe application of the current input shown in FIG. 8. FIG. 9 shows thatthe magnetization vector {right arrow over (m)}₂ of the free magneticlayer FM2 reverses 180° from the initial state, which corresponds toFIG. 5A, to the final state, which corresponds to FIG. 5E. Themagnetization components (m_(X), m_(Y)) are able to switch between(−1,0) to (1,0) as shown by the present invention.

Advantages

The high speed, low power magnetic device of the present invention usesenergy only for read and write operations or logic operations. When notenergized, the information is stored without significant loss. Thus, themagnetic device of the present invention, when implemented as a memorycell, can be used as a non-volatile memory.

The non-volatile memory provided by the magnetic device of the presentinvention is suitable for many applications, such as in computers andportable electronic devices. In particular, the high speed, low powermagnetic device of the present invention provides several advantages.The performance of the high speed, low power magnetic device of thepresent invention compares favorably with flash memory and other typesof non-volatile random access memory (RAM), such as conventionalmagnetic RAM (MRAM) and ferroelectric RAM (FRAM).

The current-induced torques act only on the magnetic device that isenergized, i.e., to which a current is applied. Therefore, when multiplemagnetic devices are arranged in an array, such as in magnetic memory,the current-induced spin transfer does not produce parasiticinteractions (“cross-talk”) between the neighboring elements in thearray, unlike in conventional magnetic memories in which magneticswitching is accomplished by using magnetic fields produced by smallcurrent-carrying wires near the magnetic elements.

The method of magnetic switching by current induced torque provided bythe present invention is faster than current conventional methods thatuse magnetic fields to switch the magnetization direction of layers.Read-out and write operations of the present invention can be completedin sub-nanosecond time scales. Conventional magnetic hard drives arevery slow compared to the magnetic memory of the present invention sincethe conventional hard drives have data access times of the order ofmilliseconds.

The method of magnetic switching by current induced torque provided bythe present invention requires low power. This is especiallyadvantageous for use in portable electronic devices.

The method of magnetic switching by current induced torque provided bythe present invention is ideal for sub-micron scale devices since thelateral dimension of the magnetic device of the present invention may beless than approximately 200 nm. Therefore, the present invention isscaled to allow the fabrication of ultra-high density memory cells sothat a vast amount of information can be stored in the magnetic memoryprovided by the present invention.

The basic architecture of the high speed, low power magnetic device ofthe present invention is straightforward, and read-out and writeoperations are reliable and are less sensitive to changes intemperature. Unlike conventional magnetic memory devices, the presentinvention does not rely on stochastic (random) processes or fluctuatingfields to initiate switching events.

According to one embodiment of the present invention, multiple bits ofinformation can be stored on each device so that even more informationcan be stored in the magnetic memory.

The method of magnetic switching by current induced torque provided bythe present invention can be used for logic operations, as well as formagnetic memory devices. Since there is a threshold, which is dependenton the shape, amplitude, and period of the current pulse, for thecurrent pulse to produce a change in magnetization, current input can becombined to produce a logic function, such as an AND gate. For example,two current pulses can be combined to produce a current pulse thattraverses the device which is the sum of the two current pulses. Thepulse characteristics (shape, amplitude, and period) can be chosen suchthat each pulse individually does not switch the device, yet thecombined pulse does switch the device. Thus, this is an AND operation. ANOT operation requires simply switching the state of the device. A NOTand an AND operation can be combined to produce a NAND function, whichis a universal digital logic gate (i.e., all digital logic functions canbe constructed from NAND gates.)

There are several possible geometries and layer configurations that areprovided by the present invention. For example, an embodiment of themagnetic device of the present invention may be configured so that nonet current passes through the free magnetic layer FM2 during writeoperations. This is illustrated in FIG. 10 which shows an embodiment ofthe present invention including current source A, current source B, andlayer I2, which is a thin insulating layer made of Al₂O₃, for example.In this device, layer I2 is 0.5 to 3 nm thick and is thin enough so thatelectrons can traverse the layer by quantum mechanical tunneling.

In the device shown in FIG. 10, current pulses are applied with currentsource A to change the magnetization direction of the free magneticlayer FM2. Using current source A, current flows from FM1 to thenon-magnetic layer N1 and electron spin angular momentum is transferredto the free magnetic layer FM2 by reflection of electrons at theinterface between the non-magnetic layer N1 and the free magnetic layerFM2. The device readout is performed using current source B. The voltageis measured when a small current from B passes between the free magneticlayer FM2 and the readout layer FM3. This voltage will depend on therelative magnetization directions of the layers FM2 and FM3 so that themagnetization direction of the free magnetic layer FM2 can be determinedto read-out the device. This device has the advantage that the readoutsignal is large since the tunnel junction resistance can be large (1 Ohmto 100 kOhm). Readout signals can be in the range from 10 mV to 1 V.

Method of Making Thin Films of High Magnetic Anisotropy MaterialMagnetized at Substantially Non-Zero Angles Relative to the Film Normal

A method of making thin films of high magnetic anisotropy material withmagnetizations oriented at substantially non-zero angles relative to thefilm normal is illustrated in FIG. 16. In FIG. 16, a deposition sourceand a substrate are provided in a vacuum chamber. The deposition sourceemits high magnetic anisotropy materials that travel to the substrate ata substantially non-zero angle θ relative to the normal of the plane ofthe substrate. The deposition source may be an evaporation source, asputtering source, or any other source suitable for depositing highmagnetic anisotropy materials on the substrate. Applicants'experimentation has revealed that the magnetization direction of theresulting thin films can be controlled by varying the angle θ betweenthe direction of deposition and the normal to the plane of thesubstrate.

The surface of the substrate is provided with a seed layer. The seedlayer may include transition metals, such as Ta, Pt, Ti, Cu or Ru,either alone or in combination. One purpose of the seed layer is to givea preferred crystalline orientation for the films that are deposited onthe substrate. Thus, the resulting films may be polycrystalline, and mayhave a preferred crystalline orientation. The substrate need not becrystallographically matched to the deposited films. Thus, a largevariety of materials may be used for the substrate, such as Si, glass,GaAs, SiN, MgO, sapphire or diamond.

In some embodiments, the deposition source provides multilayers of highmagnetic anisotropy materials to the seed layer of the substrate. In anembodiment, the individual material layers that make up the multilayersmay each have thicknesses in the range of 0.1 to 1.5 nm, and the totalmultilayer thickness may be in the range of 2 to 15 nm, although otherthicknesses may also be used. Many classes of high magnetic anisotropymaterials may be used, such as multilayers of Ni/Co, Pt/Co/Ni, orPd/Co/Ni. Another material that may be used is the L1₀ phase of FePt,which has one of the highest magnetic anisotropies among allcurrently-known materials. Magnetic layers of FePt may be constructedwith diameters as small as 3 nm while remaining thermally stable at roomtemperature, making such layers ideally suited for use in very highdensity data storage applications. Other materials that may be usedinclude, but are not limited to, multilayers of Fe and Pd, Co and Pt, orCo and Pd.

In one illustrative embodiment, the structure of the thin films is:substrate/Ta (3 nm)/Pt (3 nm)/[Co (0.1 nm)/Ni (0.6 nm)]×5/Pt (3 nm). Inthis embodiment, the seed layer is the 3 nm layer of Ta. The magneticlayer with high magnetic anisotropy is the Co/Ni layer, which isrepeated 5 times. The Co/Ni layers are surrounded by two 3 nm layers ofPt. As discussed above, the magnetization direction of such anembodiment can be selected by varying the angle between the direction ofdeposition and the normal to the plane of the substrate.

Magnetic Devices Including Layers with Magnetizations Oriented atSubstantially Non-Zero Angles Relative to the Layer Normal

In some embodiments of the present invention, a magnetic layer with amagnetization oriented at a substantially non-zero angle relative to thelayer normal is incorporated into a magnetic device configured for spintransfer switching. The magnetic layer with a magnetization oriented ata substantially non-zero angle relative to the layer normal may beconstructed by deposition of thin films of high magnetic anisotropymaterials at an angle to a substrate, as discussed above. However, thismagnetic layer may also be constructed by other means.

In some embodiments, the magnetic device includes both free and pinnedmagnetic layers, one of which has a magnetization oriented at asubstantially non-zero angle relative to the layer normal. The freelayer may have a thickness in the range of 2 to 5 nm, and the pinnedlayer may have a thickness in the range of 8 to 15 nm, although otherthicknesses may also be used. The free and pinned layers may beseparated by an insulating layer that is sufficiently thin to allowelectrons to traverse the layer by quantum mechanical tunneling, or by anonmagnetic conductor. In the former case, the device is a magnetictunnel junction (MTJ) device, and the device resistance is determined bythe tunnel magnetoresistance effect (TMR). In the latter case, thedevice is a giant magnetoresistance (GMR) device. Application of acurrent pulse of sufficient amplitude and duration causes themagnetization of the free layer to switch between stable states, therebychanging the resistance of the magnetic device. The largest change inresistance occurs when the magnetization of the free layer is switchedbetween parallel and antiparallel arrangements. However, even if themagnetization of the free layer does not switch between parallel andantiparallel arrangements, the change in resistance can still be verylarge provided that there is a large change in the projection of themagnetization of the free layer on the magnetization of the pinnedlayer. Therefore, magnetic devices in which the magnetization of thefree layer switches between states that are not parallel andantiparallel can still have large readout signals. Consequently, suchdevices are well-suited for use as magnetic memory devices.

FIGS. 15A-15D illustrate embodiments of magnetic devices in which eitherthe pinned or free layer has a magnetization oriented at a substantiallynon-zero angle relative to the layer normal. In all of FIGS. 15A-15D,the nonmagnetic layer I1 between the pinned layer FM1 and the free layerFM2 may be either an insulator or a nonmagnetic conductor. FIG. 15Aillustrates an embodiment of a magnetic device in which themagnetization {right arrow over (m)}₁ of the pinned magnetic layer FM1is oriented at a substantially non-zero angle relative to the normal ofthe pinned layer, while the magnetization {right arrow over (m)}₂ of thefree magnetic layer FM2 is, in a stable state, parallel to the normal ofthe free layer. FIG. 15B illustrates an embodiment of a magnetic devicein which the magnetization {right arrow over (m)}₂ of the free magneticlayer FM2 is, in a stable state, oriented at a substantially non-zeroangle relative to the normal of the free layer, while the magnetization{right arrow over (m)}₁ of the pinned layer is parallel to the normal ofthe pinned layer. FIG. 15C illustrates an embodiment of a magneticdevice in which the magnetization {right arrow over (m)}₁ of the pinnedmagnetic layer FM1 is oriented at a substantially non-zero anglerelative to the normal of the pinned layer, while the magnetization{right arrow over (m)}₂ of the free magnetic layer is, in a stablestate, in the plane of the free layer. FIG. 15D illustrates anembodiment of a magnetic device in which the magnetization {right arrowover (m)}₂ of the free magnetic layer FM2 is, in a stable state,oriented at a substantially non-zero angle relative to the normal of thefree layer, while the magnetization {right arrow over (m)}₁ of thepinned layer is in the plane of the pinned layer.

Structure of an Annular Magnetic Device

A pillar-shaped magnetic device 1100 having a closed periodic structureis illustrated in FIG. 11. Magnetic device 1100 includes a free magneticlayer 1110, a non-magnetic layer 1120, and a reference magnetic layer1130. The reference layer 1130 preferably has a fixed magnetic helicity1135, a magnetic vector with a fixed direction at a predetermined angle,for example, perpendicular to the plane of the layer, or both a fixedmagnetic helicity 1135 and a magnetic vector having a fixed direction ata predetermined angle. The free magnetic layer 1110 preferably has afree magnetization helicity 1115. The reference layer 1130 preferablyacts as a source of spin angular momentum. The free layer 1110 and thereference layer 1130 are preferably separated by non-magnetic layer1120.

The reference layer 1130 is preferably magnetically harder than the freelayer 1110 and preferably has a well-defined magnetic state. Thisproperty can be achieved, for example, by using a layer that is thickerthan the free layer or a material having a larger magnetic anisotropythan the material of the free layer 1110, such as Cobalt, the L10 phaseof FePt or FePd, or layered structures of Cobalt and Nickel.Alternatively, the desired hardness can be achieved through exchangecoupling to a thin anti-ferromagnetic layer, such as IrMn or FeMn.

The non-magnetic layer 1120 preferably conserves spin-momentum of thereference magnetic layer 1130 during spin transport across thenon-magnetic layer 1120. Thus, the spin diffusion length of the materialused in the non-magnetic layer 1120 is preferably longer than thethickness of the non-magnetic layer 1120. Examples of materials thatsatisfy the desired properties include any of the noble metals (e.g.,Cu, Ag, Au). The non-magnetic layer may also be an insulator such asAl₂O₃ or MgO. For a sufficiently thin insulating layer the spintransport will occur by electron tunneling so as to form a magnetictunnel junction.

The free magnetic layer 1110 preferably includes a soft magneticmaterial having a large exchange length, such as permalloy, cobalt,nickel, iron, boron and alloys of those materials. Additionally, alloysincluding non-magnetic elements, such as copper, may advantageouslyreduce the magnetic moment of the layers. Alternatively the freemagnetic layer can include magnetic oxides such as CrO₂ or Fe₃O₄.

As illustrated in FIG. 11, each layer of the magnetic device 1100 ispreferably ring-shaped (i.e. annular). An annular shape can minimize thenumber of edges or sharp corners that may act as magnetic nucleationsites which reduces stability by increasing the rate of reversal ofmagnetic helicity. A symmetrical ring structure is one of the preferredshapes which can be used to avoid unwanted reversal of helicity, howeverthe present invention may employ many various forms of closed periodstructures which may provide similar advantages. The lower therotational symmetry of the shape of the device, the more likely it isthat certain regions will be favored for magnetic nucleation andreversal of magnetic helicity. Geometries that include sharp cornersprovide strong nucleation sites that encourage helicity reversal and arepreferably avoided.

Typically devices that are known in the art result in a tradeoff betweenthe stability of the stored information represented by the free magneticlayer helicity 1115 and the speed and power requirements of changing theinformation. Typically, as the stability of the programmed helicityincreases, so does the power required to change the helicity.

Ring geometries may provide very stable magnetization orientations, withmagnetization stability for periods greater than 10 years beingachievable. Additionally, the magnetization reversal mechanism of ringgeometries may be weakly dependent on ring diameter beyond a criticallysmall size (e.g., typically tens of nanometers). Thus, the size of thedevice may not be a critical a factor when compared with most presentlyused geometries. Thus, ring geometries may enable a greater range of useand decreased production costs.

Several factors play a role in the stability of the magnetization of aring. One factor may be the size of the ring. For a given magneticfield, there exists a critical ring radius for which a ring having aradius equal to or greater than the critical size, the stability of themagnetization of the ring is relatively independent of the ring size.The stability of the magnetization may decrease rapidly as the size ofthe ring decreases below the critical size. Additionally, themagnetization may be susceptible to thermal fluctuations and theapplication of a destabilizing magnetic field.

In the limit of low noise, the rate Γ of thermally-induced transitionsbetween two stable helical magnetic states in an annular magnetic deviceis given by the Arrhenius formula:Γ˜Γ₀exp(−U/k _(B) T)  (1)where U is the energy barrier to transition, k_(B) is Boltzmann'sconstant, T is the temperature, and Γ₀ is a rate prefactor on the orderof inverse ferromagnetic resonance frequency (˜10⁻⁹ s), as calculated inK. Martens, D. L. Stein and A. D. Kent, “Magnetic reversal in nanoscopicferromagnetic rings,” Physical Review B, vol. 73, no. 5, p. 054413(2006) (hereinafter “Martens”). In order to minimize undesiredthermally-induced reversal, such that 1/Γ>>10 years, an energy barrierof U>60k_(B)T is desirable.

In Martens, the energy barrier U was calculated as a function ofmaterial parameters, ring dimensions and the applied circumferentialmagnetic field. Key parameters are the normalized magnetic field h andthe ring size l:

$\begin{matrix}{h = {\frac{H_{e}}{H_{c}} = \frac{H_{e}}{\frac{M_{0}}{\pi}\left( \frac{t}{\Delta\; R} \right){{\ln\left( \frac{t}{R} \right)}}}}} & (2) \\{l = {\frac{R}{\lambda}\sqrt{2{\pi\left( \frac{t}{\Delta\; R} \right)}{{\ln\left( \frac{t}{R} \right)}}}}} & (3)\end{matrix}$Here, M0 is the saturation magnetization, t is the ring thickness, ΔR isthe ring width, R is the average radius, λ is the exchange length, He isthe external magnetic field, and Hc is the field at which the metastablestate becomes unstable. The exchange length λ is given by λ=√{squareroot over (2 A/(μ₀M₀ ²))}, where A is the exchange constant. 1represents the ratio of the ring size to the width of a Bloch wall. Thecritical radius, i.e. the radius below which the stability of themagnetization may rapidly decrease, is the radius for which l≈2π.Setting l≈2π in Equation 3, the critical radius is given by thesolutions to:

$\begin{matrix}{{2\pi} \approx {\frac{R}{\lambda}\sqrt{2{\pi\left( \frac{t}{\Delta\; R} \right)}{{\ln\left( \frac{t}{R} \right)}}}}} & (4)\end{matrix}$The critical radius is approximately the optimal ring radius forspin-torque transfer operation because it achieves an approximatelyoptimal balance between a small ring size and a high magnetizationstability.

For l≦2π√{square root over (1−h²)}, the theory predicts a constantsaddle, as illustrated in FIG. 17A, which depicts a constant saddleconfiguration for h=0.2. By contrast, for l>2π√{square root over(1−h²)}, the theory predicts an instanton saddle, as shown in FIG. 17B,which depicts an instanton saddle configuration for l=12, and FIG. 17C,which depicts an instanton saddle configuration for l=60. Both of thesesaddle configurations are described by a function φ_(h,l)(θ), as setforth in detail in Martens.

The scale of the energy barrier U is given by:

$\begin{matrix}{E_{0} = {\frac{\mu_{0}M_{0}^{2}}{\pi}\frac{\Delta\; R}{R}{lt}\;\lambda^{2}}} & (5)\end{matrix}$

For the constant saddle arrangement, the theory provides that the energybarrier U is given by:U=E ₀(1−h)² l/2=μ₀ M ₀ ² t ² R|ln(t/R)|(1−h)²  (6)This expression is independent of the exchange length λ because thetransition state has a magnetization at a constant angle to the ringcircumference, as depicted in FIG. 17A.

For instant on saddle arrangements, the result is generally morecomplicated (see Eq. 13 of Martens). However, in the limit l>>2π, theenergy barrier for instanton arrangements is:U=4E ₀(√{square root over (1−h)}−hsec⁻¹ √{square root over (h)})  (7)This can easily reach values greater than 60k_(B)T at room temperaturefor rings fashioned from, among other materials, permalloy or CoFeB. Byway of example, a permalloy ring (A=1.3×10⁻¹¹ J/m, M₀=8×10⁵ A/m), withR=50 nm, ΔR=20 nm and t=2 nm is in the large-l, instanton limit(l=12.6), and has an energy barrier of U=80k_(B)T at room temperaturewith h=0. Therefore, such a ring would be stable against thermalfluctuations for at least 10 years, as discussed above. Rings with evenlarger ring sizes l have even greater energy barriers to reversal andare therefore easily capable of stable, long-term data retention.

FIG. 18 shows the dependence of the energy barrier U on ring radius forvarious ring thicknesses, in a situation with zero applied magneticfield (i.e., h=0). In the plot, the dashed curve represents a ring withthickness t=2 nm, the solid curve represents a ring with thickness t=3nm, and the dotted curve represents a ring with thickness t=4 nm. Theplot assumes a ring width of ΔR=0.4R and a temperature of 300 K; theenergy barrier U is plotted in units of k_(B)T. FIG. 18 shows that theenergy barrier to reversal, and therefore the magnetization stability,increases with both ring thickness and ring radius.

Utilizing these properties, a ring-shaped magnetic device can bedesigned for which the magnetization of the device is generally stableunder static operating conditions, but can easily be changed or reversedby applying a current pulse to the device. Specifically, for a ringdevice that is substantially near the critical size with no appliedcurrent, the magnetic helicity can be easily reversed by applying anelectrical current. The electrical current has the effect of providing adestabilizing field and effectively changing the value of the criticalradius of the ring. Thus, a magnetic ring designed near the criticalsize is stable and does not experience unwanted reversal under normaloperating conditions, but can be reversed by the application of arelatively small current.

Neglecting the Oersted field (i.e. setting h=0), the current I_(T) thatis required to switch the magnetization direction can be estimated as:

$\begin{matrix}{I_{T} = {\frac{e\;\alpha\; E_{0}l}{\hslash\; P}\left( {d + 1} \right)}} & (8)\end{matrix}$where e is the fundamental charge, a is the Gilbert damping constant, his the reduced Planck's constant, and P is the spin polarization of theapplied current. Furthermore, d is the ratio of the out-of-planeanisotropy to the in-plane anisotropy of the ring, given by:

$\begin{matrix}{d = \frac{2\pi^{2}R^{2}}{l^{2}\lambda^{2}}} & (9)\end{matrix}$which is typically much larger than 1. For example, a permalloy ringwith R=50 nm, ΔR=20 nm and t=2 nm has d=10.

By way of example, a permalloy ring with the above characteristics (R=50nm, ΔR=20 nm and t=2 nm) and also with α=0.01 and P=0.4 has a switchingcurrent threshold of I_(T)=440 μm and a current density threshold ofJ_(T)=6×10⁶ A/cm². This current produces a circular Oersted field ofH_(e)=1350 A/m, corresponding to h=0.02. Therefore, the Oersted fieldis, in fact, negligible in this example, and the spin-torque interactionis much more effective than the Oersted field at switching themagnetization direction.

The performance of spin-transfer devices can be evaluated by consideringa figure of merit ε defined as the ratio of the threshold current to theenergy barrier for reversal of the magnetization direction:ε=I _(T) /U  (10)Smaller values of the figure of merit ε indicate better deviceperformance because a small value of ε suggests a relatively high valueof U, and thus relatively high stability, and/or a relatively low valueof I_(T), and thus relatively low power consumption. Reducing thedamping and/or increasing the spin polarization leads to lower currentthresholds and more energy efficient devices.

FIG. 19 plots the figure of merit ε as a function of the ring size l.The figure of merit ε has a constant value ε₀ throughout the constantsaddle regime, l<2π, given by (assuming h=0) ε₀=2ea(d+1)/ hP. As thering size l increases, the instanton becomes the preferred saddleconfiguration, and the figure of merit ε increases. The dot in FIG. 19indicates the transition between the constant saddle and instantonsaddle regimes. In the limit l>>2π (i.e. as the ring size becomesarbitrarily large), ε/ε₀→l/8.

In some embodiments of the present invention, annular magnetic devicesmay be used to construct a memory cell architecture. FIG. 20 illustratesa memory cell architecture comprising bit cells that include annularmagnetic devices. In FIG. 20, each bit cell includes at least onemagnetic ring and at least one transistor for current control andreadout. A voltage may be applied on the word line (WL) to address andactivate a particular element in the memory array. In some embodiments,the transistors may be CMOS transistors. The current density per unitgate width for CMOS transistors is typically 1 mA/μm. Therefore, smallerswitching currents for the annular magnetic devices permit smallerminimum feature sizes f, smaller transistors and, consequently, largerdevice integration density. The memory density of a memory architectureincluding annular devices is therefore a function of the ring size andthe switching current. By way of example, a permalloy annular devicewith R=50 nm, ΔR=20 nm, t=2 nm, α=0.01 and P=0.4 would requiretransistors with a gate length of 0.5 μm. Assuming a lateral bit size offour times the minimum feature size, 4f, this exemplary device gives abit areal density greater than 10⁷ devices/cm².

In another aspect of this invention, a magnetic ring device that has aradius greater than or equal to the critical radius can provide a verystable magnetization. Thus, if it is not a goal of the device to modifyor reverse the magnetization of the device, a magnetic ring having aradius that is greater than the critical radius, can be easily employedin read only memory in a wide range of sizes greater than the criticalsize.

FIG. 12 illustrates a magnetic ring device 1200 employed as a magneticmemory element. Preferably, the free magnetic layer 1210 has a magnetichelicity 1215 with at least two stable orientations—a clockwiseorientation and a counter-clockwise orientation. The reference layer1230 preferably has a magnetic vector having a fixed direction atpredetermined angle 1236, a fixed magnetic helicity 1235, or both afixed magnetic vector having a direction at a predetermined angle 1236and a fixed magnetic helicity 1235. Preferably, the predetermined angleof the fixed magnetic vector 1236 is substantially perpendicular to theplane of the reference layer 1230. The reference magnetic layer 1230 andthe free magnetic layer 1210 are preferably separated by a non-magneticlayer 1220.

The direction of the free magnetic layer helicity can be changed orreversed by applying an electrical pulse across the layers of magneticdevice 1200 from current source 1270. The pulse from control currentsource 1270 may initiate the reversal of the free magnetic layerhelicity 1215. The spin-momentum of the reference magnetic layer 1230may be transferred to the free magnetic layer 1210 so as to change themagnetization and induce reversal of the free magnetic layer helicity1215. An electrical pulse in one direction across the device 1200 mayset the free magnetic layer helicity 1215 in a clockwise direction, andan electrical pulse in the opposite direction may set the free magneticlayer helicity 1215 in a counter-clockwise direction.

The electrical pulse from control current source 1270 may initiate thereversal of the free magnetic layer helicity 1215. Reversal of the freemagnetic layer helicity 1215 may stop when the second stable state isreached. However, a second current pulse from control current source1270 can be used to stop the reversal of the free magnetic layerhelicity 1215 more quickly. A reference layer 1230 having an easy axis1236 (i.e., the energetically favorable direction of the spontaneousmagnetization in a ferromagnetic material) that is substantiallyperpendicular to the free magnetic layer 1210 can lead to fasterspin-transfer induced reversal of the free magnetic layer helicity 1215.

A second reference layer 1263 with fixed magnetic layer helicity 1268may be used to read-out the helicity state of the free magnetic layer.The fixed magnetic helicity can be achieved, for example, by using alayer that is thicker than the free layer or a material having a largermagnetic anisotropy than the material of the free layer 1210, such asCobalt, the L10 phase of FePt or FePd, or layered structures of Cobaltand Nickel. The second reference 1263 layer is preferably separated fromthe free magnetic layer 1210 by a non-magnetic layer 1266, which may bea thin non-magnetic metal or insulating layer. In the case of aninsulating layer, the second reference layer 1263 and the free magneticlayer 1210 form a magnetic tunnel junction. If the free magnetic layerhelicity 1215 and the second reference magnetic layer helicity 1268 arein the same direction (i.e., the magnetic helicities are both clockwiseor both counter-clockwise), the resistance across the device 1200 may begenerally smaller than if the free magnetic layer helicity 1215 and thereference magnetic layer helicity 1235 are in opposite directions,thereby differentiating between the two stable orientations of the freemagnetic layer 1210.

FIG. 13 illustrates a further embodiment of a magnetic ring device 1300employed as a magnetic memory element in accordance with the presentinvention. Preferably, the free magnetic layer 1310 has at least twostable orientations of the free magnetic layer helicity 1315—a clockwiseorientation and a counter-clockwise orientation. The reference layer1330 preferably has a fixed magnetic vector 1336 having a direction at apredetermined angle, a fixed magnetic helicity 1335, or both a fixedmagnetic vector 1336 having a direction at a predetermined angle 1336and a fixed magnetic helicity 1335. Preferably, the predetermined angleof the fixed magnetic vector 1336 is substantially perpendicular to theplane of the reference layer 1330. The reference magnetic layer 1330 andthe free magnetic layer 1310 are preferably separated by a non-magneticlayer 1320.

The direction of the free magnetic layer helicity can be changed orreversed by applying an electrical pulse across the layers of magneticdevice 1300 from control current source 1370 through write contact 1350and contact 1340. The pulse from control current source 1370 mayinitiate the reversal of the free magnetic layer helicity 1315. Thespin-momentum of the reference magnetic layer 1330 may be transferred tothe free magnetic layer 1310 so as to change the magnetization andinduce reversal of the free magnetic layer helicity 1315. An electricalpulse in one direction across the device 1300 may set the free magneticlayer helicity 1315 in a clockwise direction, and an electrical pulse inthe opposite direction may set the free magnetic layer helicity 1315 ina counter-clockwise direction.

The electrical pulse from control current source 1370 may initiate thereversal of the free magnetic layer helicity 1315. Reversal of the freemagnetic layer helicity 1315 may stop when the second stable state isreached. However, a second current pulse from control current source1370 can be used to stop the reversal of the free magnetic layerhelicity 1315 more quickly.

A reference layer 1330 having an easy axis (i.e., the energeticallyfavorable direction of the spontaneous magnetization in a ferromagneticmaterial) that is substantially perpendicular to the free magnetic layer1310 can lead to faster spin-transfer induced reversal of the freemagnetic layer helicity 1315.

Current injection need not be symmetric. Local injection of the currentmay be used to initiate the change in the free magnetic layer helicity1315. Transfer of spin angular momentum may serve to nucleatemagnetization reversal with current spin-polarized by the referencelayer 1330. Layer 1320 is a spin preserving non-magnetic layer such asCu, Ag, Au or a thin insulating layer such as Al₂O₃ or MgO. Smallasymmetry in the ring may facilitate nucleation and reversal throughspin-momentum transfer. Pronounced asymmetry could reduce themagnetization stability, which is undesirable.

The state of the free magnetic layer helicity can be determined bymeasuring the voltage or resistance across the device 1300. If the freemagnetic layer helicity 1315 and the reference magnetic layer helicity1335 are in the same direction (i.e., the magnetic helicities are bothclockwise or both counter-clockwise), the resistance across the device1300 may be generally smaller than if the free magnetic layer helicity1315 and the reference magnetic layer helicity 1335 are in oppositedirections.

Currently available magnetic memory devices typically require relativelyhigh currents and low impedance to write information (i.e., modify themagnetic helicity of the device), whereas readout is done with smallercurrents but requires a large readout signal. The ring geometry of themagnetic device 1300 addresses these contradicting requirements byallowing the performance of the reading and writing operations indifferent locations on the device. A write operation can be performed bycontrol current source 1370, which provides a large current, and writecontact 1350, which can be in direct contact with either the freemagnetic layer 1310 or the reference magnetic layer 1330 thus having lowimpedance. The write operation circuit is completed through contact 1340which can be placed in direct contact with either the free magneticlayer 1310 or the reference magnetic layer 1330 to complete the circuitacross device 1300.

The read operation can be performed using a separate readout circuit.Read contact 1360 can include a magnetic contact portion 1363 with afixed magnetization direction or helicity 1365 and an insulator portion1366 that separates the magnetic contact 1363 from the device 1300,thereby forming a magnetic tunnel junction with the device 1300. Aseparate readout current source 1380 can provide a smaller currentacross the device 1300 which is measured by voltage or resistancereadout 1390.

Preferably, the device has a thickness of approximately 1 to 5nanometers, a mean outer radius of approximately 20 to 250 nm and a ringwidth of approximately 8 to 100 nm.

Typical multi-element magnetic devices have strong magnetostaticinteraction between the different elements. This interaction can bedifficult to quantify or control, and thus results in problemsincreasing density and performance of the device. The present inventionmay minimize these interactions. Additionally, the device avoids theproblems of magnetic field spreading which results in superior speedwriting and readout along with reduction of error due to stray or poorlycontrolled fields.

While there has been described what are at present considered to beembodiments of the present invention, it will be understood that variousmodifications may be made thereto, and it is intended that the appendedclaims cover all such modifications as fall within the true spirit andscope of the invention.

Magnetic tunnel junctions offer the possibility of very largemagnetoresistance that can be used to read the state of a magneticmemory cell. A magnetic tunnel junction consists of two magnetic layersseparated by a thin insulating layer. The insulator is sufficiently thinthat electrons may traverse this layer by quantum mechanical tunneling.The thickness of the insulator is typically between 0.3 and 3 nm.

A large magnetoresistance will provide a large readout signal. It hasbeen shown experimentally that very large magnetoresistance can beachieved using magnesium oxide (MgO) insulating barriers. Themagnetoresistance refers to the percentage change in resistance betweenstates in which the layers are magnetized antiparallel and parallel. Amagnetoresistance of greater that 400% has been achieved recently withMgO insulating layers. With an aluminum oxide insulating layer, amagnetoresistance of about 30% has been achieved. Either of thesematerials as well as other insulators may prove useful as thenon-magnetic layers, N1 or N2.

Note that current must pass through the insulating layer during theswitching process. An exception to this is the device represented inFIG. 10, in which there is a separate electrical contact to N1. Thismeans the insulator must not be damaged in the presence of this currentor, equivalently, the voltage that appears across the junction in thepresence of the current. Thin insulating barriers typically support 1V/nm electric fields before damage, known as voltage breakdown. Thecurrent required to switch the junction must not produce electric fieldsin the junction that exceed the insulator breakdown electric field.

The pinned magnetic layer of the device may include a material with aperpendicular magnetic anisotropy. A perpendicular magnetic anisotropygives a preference for the magnetization to orient perpendicular to theplane of the layer. Thin magnetic layers are typically magnetized in thefilm plane. This orientation is usually a lower energy configuration; itreduces the layer's magnetostatic energy. To orient the magnetizationperpendicular to the plane the perpendicular magnetic anisotropy must besufficiently large compared to the magnetostatic energy of the layer.

This can be achieved with a number of different materials. For example,alloys of Fe and Pt, Fe and Pd, Co and Pt, Co and Pd, Co and Au, Co andNi. This can also be achieved by creating interfaces between dissimilarmagnetic materials or magnetic materials and non-magnetic materials. Anexample of the former, is layered structures of Co and Ni and example ofthe latter is layered structure of Co and Au or Co and Pt. An advantageof these layered materials is that they need not be crystalline;polycrystalline layers suffice.

This layer serves to spin polarize the current. The materials shouldhave good spin polarization efficiency. A disadvantage of using Pd or Ptis that these elements typically induce strong spin-scattering whichreduces the layer spin-polarization. Large layer spin-polarization isneeded for efficient device operation.

The free layer's magnetization direction switches in response to acurrent pulse. It is desirable to reduce the amplitude of this currentpulse to lower the power required for device operation. Currentrequirements are linked to the magnetization density, damping andmagnetic anisotropy of the layers. The lower the magnetization densityand magnetic anisotropy the lower the required current amplitude forswitching. The magnetization density of a magnetic is lowered if themagnetic material is alloyed with a non-magnetic material. (Of course,this only holds in a range of alloy concentrations. Eventually thematerial will become non-magnetic.)

It should be noted that the switching current amplitude and switchingtime are interdependent. For example, lower magnetization densityincreases the time to reverse the magnetization.

It may be desirable to increase the magnetization damping of the freemagnetic layer to increase the device reliability. It is the expectationthat increasing the damping would increase the parameter range forswitching. That is, the device would switches reproducibly betweenstates for a greater range of current pulse amplitudes, times andcurrent pulse shapes.

The following references are hereby incorporated by reference in theirentirety: [1] S. Yuasa et al, Applied Physics Letters 89, 042505 (2006);[2] J-M. L. Beaujour, W. Chen, K. Krycka, C-C. Kao, J. Z. Sun and A. D.Kent, “Ferromagnetic resonance study of sputtered Co|Ni multilayers,”The European Physical Journal B, 59, 475 (2007); [3] J-M. L. Beaujour,A. D. Kent and J. Z. Sun, “Ferromagnetic resonance study ofpolycrystalline Fe_(—{)1−x}V_x alloy thin films” Journal of AppliedPhysics 103, 07B519 (2008); [4] K. Martens, D. L. Stein and A. D. Kent,“Magnetic reversal in nanoscopic ferromagnetic rings,” Physical ReviewB, vol. 73, no. 5, p. 054413 (2006); and [5] G. D. Chaves-O'Flynn, A. D.Kent, and D. L. Stein, Physical Review B 79, 184421 (2009).

We claim:
 1. A magnetic device comprising: a pinned magnetic layer witha first magnetization vector with a first magnetization direction thatis fixed; a free magnetic layer with a second magnetization vector witha second magnetization direction that is changeable and has four stablemagnetic states; a first non-magnetic layer spatially separating thefree magnetic layer and the pinned magnetic layer; a read-out magneticlayer with a third magnetization vector with a third magnetizationdirection that is fixed; and a second non-magnetic layer that spatiallyseparates the free magnetic layer and the read-out magnetic layer,wherein application of current pulses traversing each of the layers ofthe magnetic device switches the magnetization vector of the freemagnetic layer between the four stable magnetic states.
 2. The magneticdevice of claim 1, wherein the second magnetization direction representstwo bits of information.
 3. The magnetic device of claim 1, wherein theread-out magnetic layer is aligned such that each of the four stablemagnetic states of the free magnetic layer has a different resistance.4. The magnetic device of claim 3, wherein an in-plane component of theread-out magnetic layer magnetization vector is not parallel to oroffset by 45 degrees from any of the four stable magnetic states of thefree magnetic layer.
 5. The magnetic device of claim 1, wherein thesecond non-magnetic layer is configured to minimize mutual magneticinteraction between the free magnetic layer and the read-out magneticlayer.
 6. The magnetic device of claim 1, wherein the firstmagnetization direction is perpendicular to a plane of the free magneticlayer, and wherein the second changeable magnetization direction isperpendicular to an axis extending longitudinally through the magneticdevice.
 7. The magnetic device of claim 1, wherein the four stablemagnetic states of the second magnetization vector are separated by 90degrees.
 8. The magnetic device of claim 1, wherein the magnetic deviceis pillar-shaped, and wherein the pinned magnetic layer, the firstnon-magnetic layer, the free magnetic layer, the second non-magneticlayer, and the read-out magnetic layer are less than approximately 200nm laterally and each layer is approximately 1 nm to 50 nm thick.
 9. Themagnetic device of claim 1, wherein the pinned magnetic layer, the freemagnetic layer, and the read-out magnetic layer are comprised of Co, Ni,Fe, an alloy of Co and Ni, an alloy of Co and Fe, an alloy of Ni and Fe,an alloy of Co, Ni, Fe, or permalloy Ni1-xFex.
 10. The magnetic deviceof claim 1, wherein the pinned magnetic layer, the free magnetic layer,and the read-out magnetic layer are comprised of a non-magnetic metaland an alloy, wherein the alloy comprises an alloy of Co and Ni, analloy of Co and Fe, an alloy of Ni and Fe, or an alloy of Co, Ni and Fe,such that the non-magnetic metal and the alloy are ferromagneticallyordered at room temperature.
 11. The magnetic device of claim 10,wherein the non-magnetic metal comprises Cu, Pd or Pt.
 12. The magneticdevice of claim 1, wherein the pinned magnetic layer, the free magneticlayer, and the read-out magnetic layer are comprised of NiMnSb and aconducting magnetic oxide.
 13. The magnetic device of claim 12, whereinthe conducting magnetic oxide comprises CrO2 or Fe3O4.
 14. The magneticdevice of claim 1, wherein the non-magnetic layers comprise Cu, Cr, Au,Ag or Al.
 15. A method of magnetic switching, the method comprising:applying an electric current comprising a first current pulse followedby a second current pulse wherein the second current pulse is out ofphase with the first current pulse to a magnetic device comprising afirst magnetic layer with a first magnetization vector and a secondmagnetic layer with a second magnetization vector having four stablemagnetic states over a sub-nanosecond period of time; and stoppingapplication of the electric current when the second magnetization vectorchanges from one of the four stable magnetic states to another of thefour stable magnetic states.
 16. The method of claim 15, wherein themagnetization vector represents two bits based upon the rotation of themagnetization vector.
 17. The method of claim 15, further comprisingtransferring spin-momentum of the pinned magnetic layer to the freemagnetic layer while the electric current is applied.
 18. The method ofclaim 15, wherein the magnetic device further comprises a read-outmagnetic layer having a third magnetization vector.
 19. The method ofclaim 15, further comprising reading a state of the second magnetizationvector based upon the third magnetization vector of the read-outmagnetic layer of the magnetic device.
 20. The method of claim 19,further comprising: measuring a resistance through at least the read-outmagnetic layer and the free magnetic layer; and determining the state ofthe second magnetization vector based upon the resistance.
 21. Themethod of claim 15, wherein the current is applied through the pinnedmagnetic layer, the free magnetic layer, and the read-out magneticlayer.